Priming exercise speeds pulmonary O₂ uptake kinetics during supine “work-to-work” high-intensity cycle exercise

<jats:p> We manipulated the baseline metabolic rate and body position to explore the effect of the interaction between recruitment of discrete sections of the muscle fiber pool and muscle O<jats:sub>2</jats:sub> delivery on pulmonary O<jats:sub>2</jats:sub> uptake (V̇o<jats:sub>2</jats:sub>) kinetics during cycle exercise. We hypothesized that phase II V̇o<jats:sub>2</jats:sub> kinetics (τ<jats:sub>p</jats:sub>) in the transition from moderate- to severe-intensity exercise would be significantly slower in the supine than upright position because of a compromise to muscle perfusion and that a priming bout of severe-intensity exercise would return τ<jats:sub>p</jats:sub> during supine exercise to τ<jats:sub>p</jats:sub> during upright exercise. Eight male subjects [35 ± 13 (SD) yr] completed a series of “step” transitions to severe-intensity cycle exercise from an “unloaded” (20-W) baseline and a baseline of moderate-intensity exercise in the supine and upright body positions. τ<jats:sub>p</jats:sub> was not significantly different between supine and upright exercise during transitions from a 20-W baseline to moderate- or severe-intensity exercise but was significantly greater during moderate- to severe-intensity exercise in the supine position (54 ± 19 vs. 38 ± 10 s, P < 0.05). Priming significantly reduced τ<jats:sub>p</jats:sub> during moderate- to severe-intensity supine exercise (34 ± 9 s), returning it to a value that was not significantly different from τ<jats:sub>p</jats:sub> in the upright position. This effect occurred in the absence of changes in estimated muscle fractional O<jats:sub>2</jats:sub> extraction (from the near-infrared spectroscopy-derived deoxygenated Hb concentration signal), such that the priming-induced facilitation of muscle blood flow matched increased O<jats:sub>2</jats:sub> utilization in the recruited fibers, resulting in a speeding of V̇o<jats:sub>2</jats:sub> kinetics. These findings suggest that, during supine cycling, priming speeds V̇o<jats:sub>2</jats:sub> kinetics by providing an increased driving pressure for O<jats:sub>2</jats:sub> diffusion in the higher-order (i.e., type II) fibers, which would be recruited in the transition from moderate- to severe-intensity exercise and are known to be especially sensitive to limitations in O<jats:sub>2</jats:sub> supply. </jats:p>